1. Field of the Invention
This invention relates to a method for melting reduced iron by using an electric arc furnace for producing molten metal from said reduced iron. More particularly, the present invention relates to a method for controlling the electric arc furnace in the process of melting said reduced iron.
2. Description of the Prior Art
When reduced iron is melted for producing molten iron by using an electric arc furnace, molten metal is accumulated at the furnace bottom and a slag layer overlies the molten metal. Granular reduced iron is charged successively onto the slag layer in the furnace and passes downwardly through said slag layer. In the course of this passage, reduced iron is melted by the heat of slag and decomposed into slag component and iron component. The slag component mingles into the surrounding slag layer while the iron component passes through the slag layer and mixes in the molten metal.
In said processing of reduced iron, it is important to maintain the temperature of said molten metal layer substantially constant for the following reasons:
(a) If the molten metal temperature is too low, the molten metal is unable to maintain its intrinsic nature, that is, it cannot maintain the molten state and is solidified. PA0 (b) A too high molten metal temperature leads to a damage of the furnace and an energy loss. PA0 (c) If the slag temperature is too low, the reduced iron melting capacity of the slag is reduced, allowing the reduced iron to be directly (in a non-molten state) charged into the high-temperature molten metal. As a consequence, carbon and oxygen contained in the reduced iron are abruptly reacted and expanded (CO boiling) in the high-temperature molten metal, causing a danger of explosion. PA0 (d) A too high slag temperature not only leads to an excess increase of energy loss but also accelerates the damage to the furnace.
It is also imperative to keep the slag temperature substantially constant for the following reasons:
For three reasons, it is essential to maintain the temperatures of said molten metal and slag layer at the optimal levels. Actually, however, the molten metal and slag temperatures are widely variable according to the change of supply voltage, the change of ambient temperature, the change of temperature of the atmosphere in the furnace due to dust collection in the furnace, and other various factors. Hitherto, control of said temperatures has been made by adjusting the input power to the electric arc furnace or by adjusting the arc length. According to such methods, however, when it is tried to regulate the temperature of one of them, molten metal or slag, the temperature of the other is caused to change accordingly. That is, when one of them is adjusted to a proper temperature, the other is departed from its optimal temperature range.